The invention pertains to a device for blow-molding containers comprising at least one blow-molding station for molding the preforms into containers.
The device according to the invention is applicable in particular as part of a method for blow-molding containers in which preforms consisting of a thermoplastic material are heated in the area of a heating section and then transferred to a blowing device, in which the preforms are molded inside the molds by the action of a blowing pressure. A preferred application involves blow-molding machines with a rotating blowing wheel.
During the molding of containers by the action of a blowing pressure, preforms of a thermoplastic material such as of PET (polyethylene terephthalate) are sent to different processing stations inside a blow-molding machine. A blow-molding machine of this kind typically comprises a heating device and a blowing device, in the area of which the previously tempered preform is expanded to form the container by biaxial orientation. The expansion takes place with the help of compressed air, which is introduced into the preform to be expanded. The technical processing steps involved in the expansion of the preform are explained in DE-OS 43 40 291. The previously mentioned introduction of the compressed gas comprises both the introduction of compressed gas into the preform at the start of the blowing process and the introduction of compressed gas into the developing container bubble.
The basic structure of a blowing station for molding containers is described in DE-OS 42 12 583. Possible ways of tempering the preforms are explained in DE-OS 23 52 926. A device of the class in question for the blow-molding of containers and the method required to produce these containers are disclosed in DE 10 2007 009 026 A1.
Inside the blow-molding device, the preforms and the blown containers can be transported by different types of handling mechanisms. The use of transport mandrels, onto which the preforms are set, has proven to be especially reliable. The preforms can also be handled by other types of transport mechanisms. The use of grippers to handle preforms and the use of spreading mandrels, which can be introduced into the mouth area of the preform to hold it, also belong to the available configurations.
The handling of containers by the use of transfer wheels is described in, for example, DE-OS 199 06 438; in this case, the transfer wheel is arranged between a blowing wheel and a discharge section.
The handling of preforms discussed above can proceed according to the so-called two-stage method, in which the preforms are first produced in an injection-molding process, then stored temporarily, and only later brought up to temperature and blown into containers. The preforms can also be handled by what is known as the one-stage method, according to which the preforms are tempered and blown immediately after the injection-molding step.
This means that the preforms can be either blow-molded directly from the first heat or tempered to adjust their temperature and then blow-molded. These situations are described in DE 10 2007 009 026 A1.
There are various known embodiments of the blow-molding stations which are commonly used. In the case of blow-molding stations which are arranged on rotating transport wheels (rotary table principle), it is often seen that the mold carriers can open up like a book. It is also possible, however, for mold carriers to be used which shift relative to each other or which are guided in some other way. In the case of stationary blow-molding stations, which are especially adapted to accommodating several cavities for container molding, plates arranged parallel to each other are typically used as mold carriers.
During the technical process of molding the preforms and thus during their transformation into containers, the interior space of the preforms is subjected to considerable internal pressure to implement the blow-molding process. During blow-molding, internal pressures of up to 40 bars are in play. It is undesirable, however, for the threaded area of the preforms to change its shape also, because otherwise the caps used to seal the containers would not fit properly.
To avoid undesirable deformation of this type, it is already known that the threaded area can be mechanically supported on the outside. An external seal for the preform is also known, which has the effect of making the pressure essentially equal on both sides of the threaded area. The threaded area of the preform is for this purpose typically surrounded by a compensating space, which is sealed off against a support ring of the preform. As a result, the blowing pressure acts on all sides of the threaded area, and thus a pressure gradient between the internal and external surfaces of the threaded area which would promote deformation is avoided.
When a seal against the support ring is provided, it is necessary to prevent damage to the support ring of the preform; in addition, a reliable seal must be guaranteed even when manufacturing tolerances occur; aging phenomena of the structural elements used and temperature-related expansions of the material must also be taken into account. DE 10 2007 009 026 A1 offers a device for dealing with this list of requirements.
Accordingly, what is realized is a support element in the area of the blowing device, which is moved against the support ring of the preform, and according to which the support element, after making contact with the support ring, is first pretightened mechanically against the support ring and then tightened even further pneumatically.
The sealing element for sealing the preform extends in the area of a compensating space surrounding a mouth section of the preform, proceeding from a stop of a connecting element toward an insertion opening of the connecting element, and comprises at least one concave area, which is oriented opposite a support surface of the stop element.
Because the area of the mouth section of the preform is sealed off by the use of the sealing element, it is possible to build up a pressure outside the mouth section which prevents the mouth section from being deformed during the blow-molding process. This is realized by the uniform pressure present all the way around the mouth section of the preform.
The mechanical pretightening of the sealing element provides sufficient leak-tightness to permit the further pneumatic tightening of the seal in a second method step. The pneumatic tightening makes it possible in particular to increase the seal-producing tightness of the sealing element as the pressure against which the seal is to be produced increases.
The disadvantage of this known device is that, to realize the sealing arrangement inside the blow-molding device, the blowing nozzle must have a large diameter. As a result of the considerable amount of space which is therefore taken up and because several blow-molding stations are arranged simultaneously on the blowing device a predetermined distance apart, it is often no longer possible for gripping mechanisms for handling the preforms or the blown containers to be moved into position in the area of the preforms or containers in every position of the blow-molding stations.
Another disadvantage of this known device is that, because of the pneumatic tightening and as a result of the distance by which the sealing element extends in the direction opposite to the connecting element, it is possible for relative movement to occur between the sealing element and the concentric cylindrical surface of the mouth section supporting the sealing element.
Especially because there is great deal of static and dynamic friction between the material of the sealing element and the material of the cylindrical surface selected here, there is the danger that the remaining axial force acting on the concentric sealing surface is not sufficient to generate the two-dimensional pressure on the sealing seat necessary for a reliable sealing effect.